We propose to collect native and derivative data from crystals of two enzymes that form part of the purine salvage pathway of the parasitic protozoan Tritrichomomas foetus. One of these enzymes, Inosine Monophosphate Dehydrogenase (IMPDH) catalyzes the NAD-dependent oxidation of inosine monophosphate (IMP) to xanthine monophosphate (XMP). The structure of this enzyme will be the first IMPDH structure determined and it will serve as a model in determining the structures of both of the human IMPDH isoforms. Under a variety of circumstances, IMPDH represents an attractive target for the design of specific inhibitors which are likely to be very effective in the treatment of cancer, in preventing tissue rejection, as an antiviral agent, and in the control of parasitic infections of humans and livestock.